Does this happen in every map/scenario? I haven't come across this yet - this is running at "Balanced":

and here are my settings:

I'm only clocked to 4.0GHz, but it is after all a Ryzen 7 1800X; I'm also on Radeon RX Vega 64, but CS:GO is more dependent on the CPU unless you push above the 1080p range; either way it's on balanced and pushing well above my monitors range.

I don't know what settings you've set for your GPU, or what OS you're running.

That 112fps was one of the lowest "dips" during a 30 minute game-play recording. The next lowest dips were random 70'ish fps during the loading screens/menu. As Kingfish noted above, there are a lot of issues with CS:GO; tweaks and updates are to be expected on new platforms (Vega) and from what I've seen not many games were prepared for RX Vega. Give it some time and be on the look out for driver updates.